What is a primary key constraint?

• A. A primary key constraint requires that at least one null value is allowed.
• B. A primary key constraint enforces that the key column(s) contain unique, non-null values for each row.
• C. A primary key constraint ensures duplicates in the key are allowed.
• D. A primary key constraint governs foreign key relationships.

Answer: (B)

The primary concept here is that a primary key constraint ensures data integrity by making the key column(s) uniquely identify each row and not allow nulls. This means every row has a distinct, non-empty value for the key, which lets the database quickly locate a row and use that key as a reference point. Because of this uniqueness and non-null requirement, duplicates are prevented and missing values are not allowed, keeping the data reliable and easy to relate to other tables. The primary key is often the target that foreign keys reference, enabling relationships, but the constraint’s core job is to enforce unique, non-null identifiers for every row.